Method of lowering dew point of amibient gas within annealing furnace, device thereof, and method of producing cold-rolled annealed steel sheet

ABSTRACT

Part of an ambient gas in a heating zone and/or a soaking zone is sucked out and is cooled through a high-temperature gas passage of a heat exchanger by heat exchange with a gas in a low-temperature gas passage, further cooled by mixing with part of an ambient gas in a cooling zone, further cooled through a gas cooler, dehumidified to a dew point of −45° C. or less in a dryer, heated through the low-temperature gas passage of the heat exchanger by heat exchange with a gas in the high-temperature gas passage, and returned to the heating zone and/or the soaking zone.

TECHNICAL FIELD

This disclosure relates to advantageous production of a steel strip that can lower the dew point of an ambient gas in a continuous annealing furnace and has high wettability and, in particular, relates to a method of lowering the dew point of an ambient gas in an annealing furnace, a device for the method, and a method of producing a cold-rolled and annealed steel sheet.

BACKGROUND

It is known that when the dew point of an ambient gas in a continuous annealing furnace is −45° C. or less, surface segregation of Mn during annealing can be suppressed, and the adhesion of zinc or zinc alloy plating after annealing is improved (see Tetsu-to-Hagané (Bulletin of the Iron and Steel Institute of Japan), 96-1 (2010), pp. 11-20).

The following are examples of a known method to lower the dew point of an ambient gas in a continuous annealing furnace:

-   -   A: A method of supplying another ambient gas having a low dew         point from the outside of a furnace to a heating zone or a         soaking zone (see Japanese Unexamined Patent Application         Publication No. 2002-3953).     -   B: A method of providing a mechanism that circulates a furnace         ambient gas in the outside of the furnace and thereby performing         heat exchange between the circulating high-temperature ambient         gas and a room-temperature ambient gas having a low dew point,         which is to be separately supplied to the furnace (see Japanese         Unexamined Patent Application Publication No. 62-290830).     -   C: A method of performing heat exchange between a         high-temperature furnace ambient gas and an ambient gas having a         dew point that has been lowered in the outside of a furnace and         lowering the dew point with a water adsorption filter (see         Japanese Unexamined Patent Application Publication No.         11-124622).

In accordance with JP '953, the low-temperature gas is directly introduced into the high-temperature furnace. Thus, a large amount of thermal energy is required to maintain the steel strip temperature in the furnace, the gas temperature cannot be controlled, and the energy efficiency is very low.

In accordance with JP '830, even when the low-temperature gas has a low dew point, the low-temperature gas is mixed with a large amount of ambient gas having a high dew point in the furnace. Thus, the dew point of the ambient gas in the furnace cannot be sufficiently lowered.

In accordance with JP '622, the temperature of a gas to be returned to the furnace is not sufficiently increased and, as described in JP '622, the water adsorption filter having a low dehumidification capacity lowers the dew point only to approximately −30° C. and cannot lower the dew point to −45° C. or less. Thus, known techniques to lower the dew point of the atmosphere of a continuous annealing furnace have problems that they cannot achieve a low dew point of −45° C. or less and that they have very low energy efficiency.

SUMMARY

We provide a means of installing a dryer, for example, of a desiccant method or a compressor method that allows a dew point of −45° C. or less to lower the dew point of an annealing furnace ambient gas and a circulator to lower the dew point to −45° C., installing a heat exchanger in the circulator to increase or decrease the temperature of the gas, and modifying a gas inflow (gas introduction) into a heating zone and a cooling zone of the furnace to improve energy efficiency.

We provide in particular:

-   -   (1) A method of lowering the dew point of a furnace ambient gas         in a continuous annealing furnace for annealing a metal strip in         a lowering atmosphere by passing the metal strip through a         heating zone and a cooling zone in this order or through a         heating zone, a soaking zone, and a cooling zone in this order,         including:         -   a step (a) of providing a circulator that includes a heat             exchanger for heat exchange between a low-temperature gas             and a high-temperature gas, a gas cooler for cooling a gas,             and a dryer for dehumidifying a gas to a dew point of             −45° C. or less;         -   a step (b) of sucking part of the ambient gas from the             heating zone and/or the soaking zone;         -   then a step (c) of passing the sucked part of the ambient             gas through a high-temperature gas passage of the heat             exchanger and decreasing the temperature of the sucked part             of the ambient gas by heat exchange with a gas in a             low-temperature gas passage;         -   then a step (d) of mixing the part of the ambient gas having             a decreased temperature with part of an ambient gas sucked             from the cooling zone to further decrease the temperature of             the part of the ambient gas;         -   then a step (e) of passing the part of the ambient gas             further cooled by mixing with the part of the ambient gas             sucked from the cooling zone through the gas cooler to             further cooling the part of the ambient gas;         -   then a step (f) of dehumidifying the part of the ambient gas             further cooled through the gas cooler to a dew point of             −45° C. or less in the dryer;         -   then a step (g) of passing the dehumidified part of the             ambient gas through the low-temperature gas passage of the             heat exchanger to increase the temperature of the             dehumidified part of the ambient gas by heat exchange with a             gas in the high-temperature gas passage; and         -   then a step (h) of returning the part of the ambient gas             having an increased temperature to the heating zone and/or             the soaking zone.     -   (2) The method of lowering the dew point of an ambient gas in an         annealing furnace according to (1), wherein the part of gas         flowing from the dryer toward the low-temperature gas passage of         the heat exchanger is returned directly to the cooling zone         without passing through the heat exchanger.     -   (3) A device that lowers the dew point of an ambient gas in a         continuous annealing furnace that anneals a metal strip in a         lowering atmosphere by passing the metal strip through a heating         zone 1 and a cooling zone 2 in this order or through a heating         zone, a soaking zone, and a cooling zone in this order,         comprising:         -   a gas passage including a heat exchanger 9 for heat exchange             between a low-temperature gas and a high-temperature gas, a             gas cooler 10 for cooling a gas, a dryer 11 for             dehumidifying a gas to a dew point of −45° C. or less, and a             gas mixer 20,         -   wherein the device includes:         -   a gas passage extending from the heating zone 1 and/or the             soaking zone through a gas passage 15 to a high-temperature             gas passage of the heat exchanger 9 and through the gas             cooler 10 to the dryer 11,         -   a gas passage 16 extending from the dryer 11 to a             low-temperature gas passage of the heat exchanger 9 and from             the heat exchanger 9 to the heating zone and/or the soaking             zone, and         -   a gas passage 19 extending from the cooling zone 2, the gas             passage 19 being connected to a gas passage extending from             the gas cooler 10 to the dryer 11 in the gas mixer 20.     -   (4) The device that lowers the dew point of an ambient gas in an         annealing furnace according to (3), further comprising a gas         passage 17 to return part of gas flowing from the dryer 11         toward the low-temperature gas passage of the heat exchanger 9         directly to the cooling zone through a gas distributor 13 but         without passing through the heat exchanger 9.     -   (5) A method of producing a cold-rolled and annealed steel         sheet, comprising continuously annealing a cold-rolled steel         strip, wherein         -   the dew point of an ambient gas in the continuous annealing             furnace according to (1) or (2) is lowered by the method of             lowering the dew point of an ambient gas in an annealing             furnace according to (1) or (2) during the continuous             annealing.

Part of an ambient gas in the heating zone and/or the soaking zone is sucked out and is cooled through a high-temperature gas passage of the heat exchanger by heat exchange with a gas in a low-temperature gas passage, is then further cooled by mixing with part of an ambient gas of the cooling zone, is then further cooled through the gas cooler, is then dehumidified to a dew point of −45° C. or less in the dryer, is then heated through the low-temperature gas passage of the heat exchanger by heat exchange with a gas in the high-temperature gas passage, and is returned to the heating zone and/or the soaking zone. More preferably, part of gas flowing from the dryer toward the low-temperature gas passage of the heat exchanger is returned directly to the cooling zone without passing through the heat exchanger. These can achieve a very low dew point of −45° C. or less in the annealing furnace and significantly improve energy efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of Conventional Example 1.

FIG. 2 is a schematic view of Conventional Example 2.

FIG. 3 is a schematic view of a circulation system according to Conventional Example 2.

FIG. 4 is a schematic view of Conventional Example 3.

FIG. 5 is a schematic view of a circulation system according to Conventional Example 3.

FIG. 6 is a schematic view of Comparative Example 1.

FIG. 7 is a schematic view of a circulation system according to Comparative Example 1.

FIG. 8 is a schematic view of our Example 1.

FIG. 9 is a schematic view of a circulation system according to Example 1.

FIG. 10 is a schematic view of our Example 2.

FIG. 11 is a schematic view of a circulation system according to Example 2.

REFERENCE SIGNS LIST

-   1 Heating zone -   2 Cooling zone -   3 Steel strip -   4 Roller -   5 Suction port -   6 Inlet -   7 Ambient gas pipe -   8 Circulator -   9 Heat exchanger -   10 Gas cooler -   11 Dryer (dehumidifier) -   12 Equipment for supplying another ambient gas -   13 Gas distributor -   15 Gas flow path from heating zone -   16 Gas flow path to heating zone -   17 Gas flow path to cooling zone -   18 Water adsorption filter -   19 Gas flow path from cooling zone -   20 Gas mixer -   21 Annealing furnace outlet

DETAILED DESCRIPTION

When a cold-rolled steel strip is continuously annealed and subsequently plated with zinc or a zinc alloy, adhesion of the plating depends greatly on the dew point in an annealing furnace. It is known that this results from the amount of Mn oxide on the surface of the steel strip. At a dew point in the vicinity of −10° C., Mn oxide is present within an oxide film on the surface of the steel strip and is rarely found on the surface of the steel strip. At a dew point of −45° C. or less, Mn oxide is negligibly produced. At an intermediate dew point in the vicinity of −35° C. (−15° C. to −40° C.), a large amount of Mn oxide is produced on the surface of the steel strip and inhibits the adhesion of plating. Thus, we provide the annealing furnace with a circulator equipped with a dryer that allows a dew point of −45° C. or less to achieve a very low dew point to prevent concentration of Mn oxide on the surface of the steel strip.

Attention is now focused on the temperatures of an ambient gas sucked from the furnace into the circulator (hereinafter referred to as a sucked gas) and an ambient gas introduced from the circulator into the furnace (hereinafter referred to as an introduced gas). The desired ambient gas temperature in the annealing furnace is different in a heating zone, a soaking zone, and a cooling zone. More specifically, the sucked gas is cooled to approximately room temperature in a gas cooler before entering the dryer, dehumidified in the dryer, and returned to the furnace. Thus, if a low-temperature gas is directly introduced into a high-temperature region such as the heating zone or the soaking zone, a high temperature required to anneal the steel strip cannot be maintained. For this reason, the temperature of the introduced gas from the circulator must be increased.

We employed a method of installing a heat exchanger between the furnace and the gas cooler. More specifically, a high-temperature gas sucked from the heating zone or the soaking zone of the furnace (sucked gas) is cooled in the cooler before entering the dryer. Utilizing thermal energy resulting from the temperature difference, therefore, the gas cooled in the gas cooler and dehumidified in the dryer can be heated. Thus, thermal energy discharged from the gas cooler can be effectively utilized. A high-temperature gas sucked from the heating zone or the soaking zone of the furnace is passed through the heat exchanger, cooled in the gas cooler, dehumidified in the dryer, heated in the heat exchanger, and then returned to the heating zone or the soaking zone of the furnace.

The temperature of the high-temperature gas sucked from the heating zone or the soaking zone after the heat exchange is sometimes higher than the gas temperature in the cooling zone. Thus, the gas after the heat exchange can advantageously be mixed with a low-temperature gas sucked from the cooling zone to lower energy required to further cool the gas in the downstream gas cooler.

Furthermore, since the gas temperature after cooling with the gas cooler is lower than the temperature of the cooling zone of the furnace, part of gas cooled in the gas cooler, dehumidified in the dryer, and returned directly to the cooling zone without passing through the heat exchanger can lower the temperature and the dew point of the cooling zone, thus further improving energy efficiency.

Unlike a water adsorption filter made of activated alumina, alternately operated and stopped, and having a low dehumidification capacity as described in JP '622, a dryer for use herein preferably has a high dehumidification capacity, for example, of a desiccant method for continuous dehumidification using calcium oxide, zeolite, silica gel, or calcium chloride or a compressor method using an alternative chlorofluorocarbon.

Examples

FIGS. 1 to 11 illustrate the structure and gas passages of a continuous annealing furnace having a heating zone and a cooling zone according to Examples, Comparative Example, and Conventional Examples.

FIG. 1 illustrates Conventional Example 1 described in JP '953. Ambient gas supply equipment 12 directly supplies another low-temperature ambient gas to a heating zone 1 and a cooling zone 2.

FIGS. 2 and 3 illustrate Conventional Example 2 described in JP '830. A gas sucked from a cooling zone 2 enters a circulator 8 through a flow path 15, passes through a heat exchanger 9 to heat a gas from ambient gas supply equipment 12, and returns to the cooling zone 2 through a flow path 16. The low-temperature ambient gas supplied from the gas supply equipment 12 is heated in the heat exchanger 9 and introduced into a heating zone 1 through an ambient gas pipe 7.

FIGS. 4 and 5 illustrate Conventional Example 3 described in JP '622. A gas sucked from heating zone 1 is introduced into a circulator 8 through a flow path 15, cooled in a heat exchanger 9 with a gas from a water adsorption filter 18, dehumidified with the water adsorption filter 18 made of activated alumina, heated in the heat exchanger 9, and returned to the heating zone 1 through a flow path 16. Each device includes three water adsorption filters 18 alternately operated at intervals of three hours.

FIGS. 6 and 7 illustrate Comparative Example 1. A gas sucked from a heating zone 1 is introduced into a circulator 8 through a flow path 15, cooled in a heat exchanger 9 with a gas that has been dehumidified in a dryer 11, further cooled in a gas cooler 10, dehumidified in the dryer 11, heated in the heat exchanger 9 with a gas from the heating zone 1, and returned to the heating zone 1 through a flow path 16.

FIGS. 8 and 9 illustrate our Example 1. A gas sucked from a heating zone 1 is introduced into a circulator 8 through a flow path 15, cooled in a heat exchanger 9 with a dehumidified gas from a dryer, mixed in a mixer 20 with another gas sucked from a cooling zone 2 through a flow path 19, further cooled in a cooler 10, dehumidified in a dryer 11, heated with a gas from the heating zone 1, and returned to the heating zone 1 through a flow path 16.

FIGS. 10 and 11 illustrate our Example 2. In addition to Example 1 illustrated in FIGS. 8 and 9, the gas dehumidified in the dryer 11 is distributed with a gas distributor 13. One part of the distributed gas is introduced into the heat exchanger 9, heated therein with a gas from the heating zone 1 and returned to the heating zone 1 through a flow path 16. The other part of the distributed gas is returned directly to a cooling zone 2 through a flow path 17.

The conditions of these sucked gases and introduced gases were changed. Table 1 shows the dew points of the sucked gases and the dew points of the introduced gases passing through the gas passages and exhausted heat energy during the passage in Examples and Conventional Examples. Table 1 shows that the dew points of the gases introduced into the annealing furnaces in No. 1 to No. 3 of Example 1 and No. 4 to No. 6 of Example 2 are satisfactorily lower than the target temperature of −45° C., as compared with Conventional Examples No. 7 to No. 10. Furthermore, the dew points in the furnaces measured upstream from an annealing furnace outlet 21 are also satisfactorily lower than −45° C. Furthermore, No. 1 to No. 3 of Example 1 and No. 4 to No. 6 of Example 2 exhausted less heat energy and have very high energy efficiency.

Adhesion of zinc alloy plating was examined in zinc alloy plating of a steel strip after continuous annealing in accordance with a JIS-H8504(g) tape test method (a chipping test method). As a result, Examples No. 1 to No. 6 had satisfactorily strong adhesion, but Conventional Examples No. 7 to No. 10 had coating defects.

TABLE 1 Sucked gas Introduced gas Flow Temper- Dew Flow Temper- Dew rate ature point rate ature point No. Position Nm³/Hr ° C. ° C. Position Nm³/Hr ° C. ° C. 1 Heating zone 500 800 −20 Heating zone 750 600 −52 Cooling zone 250 50 −25 2 Heating zone 500 850 −25 Heating zone 1000 500 −55 Cooling zone 500 150 −15 3 Heating zone 1500 950 −25 Heating zone 2000 700 −60 Cooling zone 500 20 −15 4 Heating zone 1000 700 −10 Heating zone 1000 550 −50 Cooling zone 1000 200 −10 Cooling zone 1000 20 5 Heating zone 500 850 −25 Heating zone 250 600 −51 Cooling zone 1500 30 −15 Cooling zone 1750 25 6 Heating zone 2000 950 −15 Heating zone 1500 600 −70 Cooling zone 500 20 −25 Cooling zone 1000 5 7 Cooling zone 0 — — Cooling zone 3000 25 −50 8 Heating zone 0 — — Heating zone 1500 5 −45 9 Heating zone 500 950 −20 Heating zone 500 700 −20 (250) 200 −40 10 Heating zone 4000 800 −15 Heating zone 4000 600 −35 Dew point in furnace measured upstream Adhesion of Zn from continuous Exhausted Dehumidi- alloy plating annealing furnace heat energy fication after continuous No. outlet (° C.) kJ/Nm³ method annealing Note 1 −50 68 Calcium Strong Example 1 oxide 2 −52 80 Zeolite Strong Example 1 3 −55 78 Silica gel Strong Example 1 4 −48 42 Zeolite Strong Example 2 5 −49 35 Calcium Strong Example 2 chloride 6 −65 40 Compressor Strong Example 2 method 7 −35 253 — Coating defect Conventional Example 1 8 −32 402 — Coating defect Conventional Example 1 9 −21 155 — Coating defect Conventional Example 2 10 −23 189 — Coating defect Conventional Example 2 [Note] A flow rate in parentheses is the flow rate of another supplied gas. 

1-5. (canceled)
 6. A method of lowering the dew point of a furnace ambient gas in a continuous annealing furnace that anneals a metal strip in a reducing atmosphere by passing the metal strip through a heating zone and a cooling zone in this order or through a heating zone, a soaking zone, and a cooling zone in this order, comprising: (a) providing a circulator that includes a heat exchanger for heat exchange between a low-temperature gas and a high-temperature gas, a gas cooler for cooling a gas, and a dryer for dehumidifying a gas to a dew point of −45° C. or less; (b) sucking part of the ambient gas from the heating zone and/or the soaking zone; (c) passing the sucked part of the ambient gas through a high-temperature gas passage of the heat exchanger and decreasing the temperature of the sucked part of the ambient gas by heat exchange with a gas in a low-temperature gas passage; (d) mixing the part of the ambient gas having a decreased temperature with part of an ambient gas sucked from the cooling zone to further decrease the temperature of the part of the ambient gas; (e) passing the part of the ambient gas further cooled by mixing with the part of the ambient gas sucked from the cooling zone through the gas cooler to further cooling the part of the ambient gas; (f) dehumidifying the part of the ambient gas further cooled through the gas cooler to a dew point of −45° C. or less in the dryer; (g) passing the dehumidified part of the ambient gas through the low-temperature gas passage of the heat exchanger to increase the temperature of the dehumidified part of the ambient gas by heat exchange with a gas in the high-temperature gas passage; and (h) returning the part of the ambient gas having an increased temperature to the heating zone and/or the soaking zone.
 7. The method according to claim 6, wherein the part of gas flowing from the dryer toward the low-temperature gas passage of the heat exchanger is returned directly to the cooling zone without passing through the heat exchanger.
 8. A device that lowers the dew point of an ambient gas in a continuous annealing furnace that anneals a metal strip in a reducing atmosphere by passing the metal strip through a heating zone and a cooling zone in this order or through a heating zone, a soaking zone, and a cooling zone in this order, comprising: a gas passage including a heat exchanger that exchanges heat between a low-temperature gas and a high-temperature gas, a gas cooler that cools a gas, a dryer that dehumidifies a gas to a dew point of −45° C. or less, and a gas mixer, a gas passage extending from the heating zone and/or the soaking zone through a gas passage to a high-temperature gas passage of the heat exchanger and through the gas cooler to the dryer, a second gas passage extending from the dryer to a low-temperature gas passage of the heat exchanger and from the heat exchanger to the heating zone and/or the soaking zone, and a third gas passage extending from the cooling zone, the third gas passage being connected to a fourth gas passage extending from the gas cooler to the dryer in the gas mixer.
 9. The device according to claim 8, further comprising a fifth gas passage that returns part of gas flowing from the dryer toward the low-temperature gas passage of the heat exchanger directly to the cooling zone through a gas distributor but without passing through the heat exchanger.
 10. A method of producing a cold-rolled and annealed steel sheet, comprising continuously annealing a cold-rolled steel strip, wherein the dew point of an ambient gas in a continuous annealing furnace is reduced by the method according to claim 6 during the continuous annealing.
 11. A method of producing a cold-rolled and annealed steel sheet, comprising continuously annealing a cold-rolled steel strip, wherein the dew point of an ambient gas in a continuous annealing furnace is reduced by the method according to claim 7 during the continuous annealing. 